This invention relates to the use of catechol or substituted catechol half esters of beta-haloethylphosphonic acid to inhibit head formation in lettuce.
Commercially important varieties of crisphead lettuce (Lactuca sativa) form heads, as they mature. Heads occur when the leaves curve over the center of the plant and fold over on each other tightly such that the exposure of the interior leaf tissue to sunlight is inhibited. Typically, these leaves are not well-pigmented and lack an abundance of chlorophyll in comparison with leaves that have been exposed to the sun. Though consumers enjoy the crisp nature of head lettuce, the lettuce lacks another highly valued consumer trait, darker green pigmentation, with a resulting higher .beta.-carotene content.
Some phosphonate compounds have found use in the agricultural art, for example, for herbicidal purposes as described in U.S. Pat. Nos. 2,927,014 and 3,223,514. Compound 2-chloroethylphosphonic acid has been described in U.S. Pat. No. 4,240,819 to be useful in inhibiting plant growth. It is currently marketed for use in tomatoes, cherries, grapes, apples, walnuts, peppers, blackberries and cantaloupes and is advocated for the use of promoting early coloration and maturity of tomatoes, grapes, apples and pears. In the case of cherries, walnuts and apples, it loosens the fruit for earlier, more efficient harvest. The compound promotes fruit abscission in cantaloupes and accelerates ripening and loosening of blackberries.